The present invention generally relates to a wireless signal detection system and more particularly relates to a flexible search system for detecting a signal transmitted via a frequency-shift keying modulation technique.
Frequency-shift keying (FSK) modulation is one form of modulation used to transmit information in wireless communication systems. In FSK modulation, the modulating signal shifts the output frequency between predetermined frequency values. For example, in binary FSK (or BFSK) two different frequencies may be used such that a transmission at one frequency indicates one symbol and a transmission at the other frequency indicates a second symbol.
In transmitting digital information, one symbol can represent a xe2x80x9c1xe2x80x9d and the other symbol can represent a xe2x80x9c0.xe2x80x9d By shifting transmission between the various frequencies at the appropriate times, a sequence of information can be communicated. The actual symbol or symbols associated with the transmission at a given frequency need not, however, be a simple xe2x80x9c1xe2x80x9d or xe2x80x9c0,xe2x80x9d but can be some other symbol or symbols as defined by the communication protocol being used.
M-ary FSK or MFSK is another related form of FSK modulation. In general, the M refers to the number of possible signals or tones that may be transmitted during each symbol period of Ts seconds. As in the preceding BFSK example, the frequencies in the constellation are mapped to binary representations. In digital communication systems, it is most efficient if the number of tones in the constellation is equivalent to M=2k, where k is any integer greater than or equal to 1 and, therefore, represents the number of bits that can be represented in the M-ary signal set. For BFSK, k is equal to 1, which is the number of bits that can be represented with each of the two transmitted tones.
One well-known form of 8-ary FSK modulation that is used in the high frequency (HF) band (3 MHz to 30 MHz) is MIL-STD-188-141A, which uses eight frequencies spaced 250 Hz apart. In this 8-ary waveform, k=log2 (8)=3. Therefore, the eight symbols (or tones in the constellation) represent all possible combinations of three bits. This waveform is used in systems that provide an automatic linking protocol between different transceivers. For this reason, it is commonly referred to as the Automatic Link Establishment (or ALE) waveform. Each transmitted tone has a duration, or period, of 8 milliseconds. The transmitted symbol rate is the reciprocal of the symbol period, which is 125 symbols per second. In the ALE waveform, each symbol represents a string of three bits, as shown in Table 1 below. The symbol indicated by each frequency is also shown in Table 1 (the frequency indicated is the frequency relative to the center frequency of the transmission):
In certain applications it is desired to determine whether a signal having certain known characteristics, such as an FSK signal, is being transmitted. In such circumstances it is necessary to search or scan the spectrum for the known characteristics. By way of example, an entity may wish to determine whether ALE signals having arbitrary channel designations are being transmitted. If it is determined that such a signal exists, then it is generally desirable that the signal be properly centered in the baseband spectrum so that the demodulator can properly demodulate the received tones. Improper channel alignment will prohibit the demodulator from properly demodulating and decoding the ALE signal.
In searching for the signal of interest, the receiver can scan (as in the case of HF, systems) in the upper side band (USB) or lower side band (LSB) of the current channel using an observation window. To search for an ALE signal, for example, the observation window should have a width of 3 kHz or more. The observation window used for the search, however, may have an alignment relative to the received signal such that only a subset of the received FSK tones fall within the scope of any observation window. The key to efficiently scanning the total range of frequencies of interest is to intelligently process the information available to the receiver. Current search methods, however, accomplish their objective by recording a very large segment of the spectrum and then analyzing the recording for:the presence of the signal pattern of interest. This type of search cannot be accomplished in real time. Other search methods incrementally scan the receiver in partial channel bandwidth increments, so as to xe2x80x9cslidexe2x80x9d the FSK signature into full view of the algorithm used to detect its presence. This is inefficient and does not take into account some of the characteristics present in typical M-ary FSK waveforms.
Consequently, there exists a need for an improved FSK search method and apparatus. A method and apparatus is needed that is capable of performing efficient, real time searches for FSK signals. Such a tool could be used by a participant in a communication network that is seeking another participant with which to communication. It can also be used, for example, by a receiving apparatus that is only interested in detecting and listening to certain FSK signals. These needs and others are fulfilled by the invention disclosed in the following detailed description.
It is an object of the present invention to provide a method and apparatus for detecting the presence of signals transmitted by frequency-shift keying modulation techniques
It is a feature of the present invention to utilize a search technique capable of identifying a frequency-shift keyed signal even if only a portion of the signal is located within the search window, often due to an arbitrary channel designation, and then centering the detected signals for proper demodulation.
It is an advantage of the present invention to provide the capability of efficient real-time detection of signals transmitted via frequency-shift keying modulation techniques.
Thus, the present invention involves an apparatus and method for detecting a wirelessly communicated signal having certain known characteristics. It can be carried out in a xe2x80x9cdelay-lessxe2x80x9d manner in the sense that the search for such a signal be performed in real time. Further, the algorithm will make guesses as potential signatures are detected to further increase the efficiency of the scanning process. The invention can be used for detecting the presence of virtually any type of FSK signal. In one embodiment, for example, the invention can be used to detect the transmission of an ALE signal.
For example, when using a scanning radio to locate a FSK signal, arbitrary frequency offsets are possible and expected. The present invention provides an intelligent method for determining (possibly with limited information) the potential location of a FSK signature. Although the present invention is ideal for applications involving higher-order constellations, it is certainly not limited to such applications. Accordingly, the present invention is applicable to a wide range of FSK search applications.